The ability to accurately locate the position of an aircraft detected by a radar system is an important capability of such systems. In particular, due to the speed at which modern aircraft operate, it is especially important for mid-air collision avoidance and in the control of landing approaches in inclement weather. The control of aircraft in a specified region of air space is referred to as air traffic control (ATC).
ATC systems often utilize a system referred to as the air traffic control radar beacon system (ATCRBS) which is used by ATC to enhance surveillance radar monitoring and separation of air traffic. ATCRBS assists ATC surveillance radars by acquiring information about the aircraft being monitored and providing this information to the radar controllers. The controllers can then use the information to identify radar returns from aircraft which are referred to as targets and to distinguish those returns from ground clutter. The aircraft's information is contained in the returned signal from a transponder located on the aircraft.
An ATC ground station usually includes two radar systems. The first is the primary surveillance radar (PSR). The second is the secondary surveillance radar (SSR). The primary surveillance radar operates by transmitting radio pulses, listening for any reflections from aircraft in its detection zone, and measuring the time between those transmitted pulses and their reflected pulses. The SSR depends on a cooperating transponder installed on an aircraft being tracked. The transponder emits a signal when it receives a signal from the secondary radar. The transponder can send encoded information about the aircraft, such as identity and altitude.
Errors in the locations of targets by radar systems such as ATC systems are compensated for by registration techniques. Current processes for accurately registering a radar system use an accurately surveyed transponder, referred to as a Position Adjustable Range Reference Orientation Transponder (PARROT). By comparing measured PARROT range and azimuth values to values derived from the surveyed position of the PARROT and the surveyed position of the radar, a bias or offset error in the radar system between the two can be calculated and then compensated for. The process involves measuring the PARROT range and azimuth data over multiple scans and applying proper filtering to achieve the required registration accuracy.